In the lorry rear wheel drive system shown in FIGS. 1 and 2, a central support bearing unit 100 is fixed beneath the vehicle body by means of a fastening means (not shown), to provide radial support for a drive shaft (propeller shaft) 200. In the lorry rear wheel drive system, the drive shaft 200 has a high rotation speed, up to 2,500 rpm. To accommodate such a high rotation speed, the central support bearing unit 100 conventionally employs a deep groove ball bearing. FIG. 3 shows a partial sectional schematic diagram of a deep groove ball bearing and a conventional seal thereof. In practice, the deep groove ball bearing in the assembly has a very high failure rate, because it is difficult for the seal thereof to block the ingress of external contaminants, and this readily causes premature failure of the bearing.
To solve this problem, the applicant once tried using the sealing scheme shown in FIG. 4. This is a cassette seal used for a vehicle wheel hub bearing, providing an inner frame (flinger or inner stamping) 2 and an outer frame (outer stamping) 4 which are adjacent to each other after installation. After assembly, the inner frame 2 forms a tight fit with an outer surface of a bearing inner ring, while the outer frame 4 forms a tight fit with an inner surface of a bearing outer ring. A sealing body 5, which helps to prevent lubricant from leaking out of the bearing interior via the inner surface of the bearing outer ring, is formed on the outer frame 4. The sealing body 5 extends towards the inner frame 2 to form an axial lip 7 and a radial lip 9 in elastic contact with the inner surface of the inner frame, for sealing an axial gap and a radial gap, respectively, between the inner frame 2 and the outer frame 4. The radial lip 9 is further provided with a garter spring 3 around a peripheral neck thereof, for providing an additional radial constraining force for the radial lip 9. In a typical situation, the garter spring 3 is a coil spring with its two ends connected together, which can provide an additional binding force for the radial lip in a contracted state. Although such a cassette seal for a vehicle wheel hub bearing has good sealing performance, it can hardly accommodate a lorry rear wheel drive system, which demands higher rotation speeds. Experiments have demonstrated that even if the garter spring 3 is removed, to eliminate the constricting action thereof on the radial lip 9, the friction effect between the radial lip 9 and the inner frame 2 is still significant, and temperature rise experiments can still hardly meet the standard requirements. Too high a temperature rise is highly likely to cause premature failure of the bearing and seal, so that the vehicle's technical requirements for the central support bearing unit cannot be met. The industry awaits a bearing seal that can meet higher rotation speeds, and which has a low temperature rise and a reliable sealing effect.